Electromechanical transducer and production method therefor

ABSTRACT

An electromechanical transducer includes a plurality cells that are electrically connected to form a unit. Each of the cells includes a first electrode and a second electrode provided with a gap being disposed therebetween. Dummy cells that are not electrically connected to the cells are provided around the outer periphery of the unit of the cells.

TECHNICAL FIELD

The present invention relates to an electromechanical transducer and aproduction method therefor.

BACKGROUND ART

In recent years, electromechanical transducers produced by amicromachining process have been researched actively. In particular,capacitive electromechanical transducers called CMUT (CapacitiveMicromachined Ultrasonic Transducers) have attracted attention, becausethey can transmit and receive ultrasonic waves with a lightweightvibrating membrane, and can more easily obtain wide band characteristicseven in the liquid and air than piezoelectric electromechanicaltransducers that have been used hitherto.

The structure of a CMUT will be described below. A CMUT includes aplurality of elements arranged in an array in a one-dimensional ortwo-dimensional direction. These elements serve to transmit or receiveultrasonic waves.

The structure of an element in the CMUT will be described below. Asshown in FIG. 11A, an element 301 of a CMUT includes a plurality ofcells 311. By simultaneously applying driving voltage signals of thecells 311 in the element 301, ultrasonic waves are output from theelement 301. Further, ultrasonic detection signals received by the cells311 in the element 301 are added by upper electrodes 315 and lowerelectrodes (not shown) that are common to the cells 311, and the sum ofthe signals serves as an ultrasonic detection signal received by theelement 301. Lines 307 electrically connect the upper electrodes 315 ofthe cells 311.

U.S. Pat. No. 6,958,255 discloses an example of a CMUT having such anelement structure. In the CMUT disclosed in U.S. Pat. No. 6,958,255, asubstrate through line 304 is provided in a support substrate 305, asshown in FIG. 11B. The substrate through line 304 electrically connectsa circuit board 303 and a lower electrode 316. In the circuit board 303,driving voltage signals are generated to output an ultrasonic wave froman element, and signal processing, such as amplification and delayaddition, is conducted on an ultrasonic signal generated by anultrasonic wave received by the element.

The structure and operation principle of the cell of the CMUT will bedescribed below. As shown in FIG. 12A, a cell 311 of the CMUT includes amembrane 312, an insulating layer 313, a cavity 314, an upper electrode315, and a lower electrode 316. The upper electrode 315 and the lowerelectrode 316 constitute a capacitor, and a bias voltage is appliedtherebetween by a bias voltage source 317. For transmission of anultrasonic wave, a driving voltage signal having a proper waveform isapplied between the upper and lower electrodes 315 and 316 by a drivingvoltage signal source 318, whereby the membrane 312 vibrates to generatean ultrasonic wave in accordance with the driving voltage signal, asshown in FIG. 12A. Conversely, for receiving, the membrane 312 isvibrated by an ultrasonic wave reaching the CMUT, whereby anelectrostatic capacitance between the upper and lower electrodes 315 and316 changes and a current signal is generated in accordance with theultrasonic wave, as shown in FIG. 12B. By detecting this current signalwith a current detector 319, the received ultrasonic wave can bedetected.

SUMMARY OF INVENTION

Unfortunately, the displacement amount of the membrane varies among thecells of the element. It can be conceived that this variation among thecells is caused by warping due to the difference in coefficient ofthermal expansion between the membrane and the insulating layer andinternal stresses in the membrane and the insulating layer. Thevariation in displacement amount among the cells is undesirable becauseit appears as differences in transmission efficiency and detectionsensitivity for the ultrasonic wave.

in particular, to normally operate the CMUT, a phenomenon called apull-in, in which the upper electrode is attracted to the lowerelectrode together with the membrane, is to be avoided. To avoid apull-in, it is necessary to set the bias voltage so that a pull-in willnot occur in a cell whose initial displacement amount is the largest.Here, the transmission efficiency and detection sensitivity of the CMUTincrease as the gap between the upper and lower electrodes decreases.Since electrostatic attractive force between the upper and lowerelectrodes is increased by increasing the bias voltage, the transmissionefficiency and detection sensitivity of the CMUT can be enhanced byincreasing the bias voltage. However, when the bias voltage excessivelyincreases, a pull-in occurs the instant that the bias voltage reaches acertain voltage, so that a desired vibration characteristic cannot beobtained. A voltage at which a pull-in occurs is referred to as apull-in voltage. A pull-in voltage is determined by the initialdisplacement amount of the membrane. Thus, since the upper limit valueof the bias voltage applied between the upper and lower electrodes islimited by variation in initial displacement of the membrane among thecells, the receiving sensitivity of the CMUT is limited.

The present invention provides an electromechanical transducer thatreduces variation in displacement amount of a membrane among cells.

An electromechanical transducer according to an aspect of the presentinvention includes an element. The element includes a plurality of cellseach including a first electrode and a second electrode provided with acavity therebetween, the cells being electrically connected in parallelto form a unit; and a dummy cell provided around an outer periphery ofthe unit of the cells, the dummy cell not being electrically connectedto the cells.

A production method according to another aspect of the present inventionis for an electromechanical transducer including an element having aplurality of cells each including a first electrode and a secondelectrode provided with a cavity being disposed therebetween, the cellsbeing electrically connected in parallel to form a unit. The productionmethod includes the step of forming a dummy cell around an outerperiphery of the unit of the cells, the dummy cell not beingelectrically connected to the cells.

According to the present invention, since variation in displacementamount of a membrane among the cells is reduced by forming dummy cellsaround the outer periphery of the unit of the cells, variation inreceiving sensitivity among the cells can be reduced in theelectromechanical transducer.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1A is a top view showing a structure of an element in a CMUTaccording to a first embodiment of the present invention.

FIG. 1B is a cross-sectional view taken along line IB-IB in FIG. 1A.

FIG. 2A is a top view showing another structure of an element in theCMUT according to the first embodiment.

FIG. 2B is a cross-sectional view taken along line IIB-IIB in FIG. 2A.

FIG. 3 shows a structure of an element in a CMUT according to a secondembodiment of the present invention.

FIG. 4A is a top view showing a structure of an element in a CMUTaccording to a third embodiment.

FIG. 4B is a cross-sectional view taken along line IVB-IVB in FIG. 4A.

FIG. 5 shows a structure of an element in a CMUT according to a fourthembodiment of the present invention.

FIG. 6 shows another structure of an element in the CMUT according tothe fourth embodiment.

FIG. 7A illustrates a CMUT production method according to the presentinvention.

FIG. 7B illustrates the CMUT production method.

FIG. 7C illustrates the CMUT production method.

FIG. 7D illustrates the CMUT production method.

FIG. 7E illustrates the CMUT production method.

FIG. 7F illustrates the CMUT production method.

FIG. 8A shows another CMUT production method according to the presentinvention.

FIG. 8B shows the CMUT production method.

FIG. 9A is a schematic view showing the initial displacement amountprovided in a case in which a dummy cell is not provided.

FIG. 9B is a schematic view showing the initial displacement amountprovided in a case in which a dummy cell is provided.

FIG. 10A is a graph showing the relationship between the depth of acavity of a dummy cell and the difference in initial displacement of amembrane so as to show the advantages of the present invention.

FIG. 10B is a graph showing the relationship between the width of thecavity in the dummy cell and the difference in initial displacement ofthe membrane.

FIG. 11A is a top view showing a structure of an element in a CMUT ofthe related art.

FIG. 11B is a cross-sectional view showing a structure of an element ina CMUT of the related art.

FIG. 12A show a structure and operation principle of a cell in the CMUTof the related art provided during transmission of an ultrasonic wave.

FIG. 12B shows the structure and operation principle of the cell in theCMUT of the related art provided during receiving of an ultrasonic wave.

DESCRIPTION OF EMBODIMENTS

The present inventors have found, from their knowledge, that variationin initial displacement amount of a membrane among cells can be reducedby forming, around the periphery of a unit of electrically connectedcells, dummy cells that are not electrically connected to the cells.

In the present invention, cells are provided in a plurality of rows, andare electrically connected in parallel to form a unit. In each cell, alower electrode serving as a first electrode and an upper electrodeserving as a second electrode are provided with a cavity being disposedtherebetween. Further, dummy cells that are not electrically connectedto the cells are provided around the periphery of the unit ofelectrically connected cells. The unit of cells and the dummy cellsconstitute an element. That is, the expression that the “dummy cellsthat are not electrically connected to the electrically connected cellsare provided around the periphery of the unit of the cells” means that“dummy cells are provided around the peripheries of cells provided onthe outermost periphery of the element, and in an arrangement mannersimilar to that of the cells”. In FIGS. 1A and 1B, cells 109 provided onthe outermost side of the element serve as “cells provided on theoutermost periphery of the element”, and cells 110 around theperipheries of the cells provided on the outermost periphery serve asdummy cells. Further, while the element shown in FIGS. 1A and 1Bincludes twenty-five cells, the electromechanical transducer of thepresent invention is not limited thereto, and the element may include adesired number of cells. In addition, while one element is shown inFIGS. 1A and 1B, an arbitrary number of elements can be provided in theelectromechanical transducer, and a plurality of elements may bearranged in a two-dimensional array.

The term “dummy cell” refers to a structure that includes at least amembrane serving as a vibrating membrane and a cavity and that is notelectrically connected to a cell (that is not used as a signal). Thedummy cell may include a lower electrode serving as a first electrodeand an upper electrode serving as a second electrode as long as it isnot electrically connected to the cell. In other words, even when anupper electrode and a lower electrode are provided in the dummy cell ofthe element, it is only necessary that one of the upper electrodes andlower electrodes in the dummy cell is electrically connected to theupper electrode or the lower electrode in the cell. With this, theoutput from the dummy cell is electrically separated from the outputfrom the unit of cells, and is not used as a signal.

In the present invention, the upper electrode can be formed by a filmmade of a choice from metal, low-resistance amorphous silicon, and alow-resistance oxide semi-conductor. The membrane may also function asthe upper electrode. The lower electrode can be formed of any materialthat has a low electrical resistance, for example, a dopedsingle-crystal silicon substrate, a doped polycrystal silicon film, asingle-crystal silicon substrate having a doped region serving as alower electrode, a doped amorphous silicon, an oxide semiconductor, or ametal film. The substrate can also function as the lower electrode.

It is conceivable that variation in displacement amount of the membraneamong the cells is reduced by the configuration of the electromechanicaltransducer of the present invention because, in peripheral edge portionsof cells on the outermost periphery of the element, structures of themembrane and the insulating layer (e.g., the joint area between themembrane and the insulating layer) are identical or considerably closeto those of the other cells. Thus, the distribution of internal stressof the membrane in the outermost cells is identical or considerablyclose to that of the other cells. Hence, it is conceivable that theeffect of reducing variation in displacement amount of the membraneamong the cells can be obtained by arranging the dummy cells.

The following is the result of verification of the above-describedeffect made by calculation using a finite element method. A model of anelement in which cells are arranged in line and a model in which dummycells are provided outside end cells were prepared, and variation ininitial displacement amount of the membrane due to the internal stressgenerated in the membrane and the insulating layer was calculated by thefinite element method. FIG. 9A shows the initial displacement amount ofthe membrane provided when a dummy cell is not provided, and FIG. 9Bshows the initial displacement amount of the membrane provided whendummy cells 110 are provided. The initial displacement amount of themembrane is the amount of displacement caused by a resultant force ofthe internal stress in the membrane and the pressure applied by thedifference in atmospheric pressure between the interior and exterior ofthe cavity (about one atmospheric pressure=101325 Pa). As the internalstress to he applied, a thermal contraction stress generated by thetemperature difference caused between the times before and afterformation of the membrane was assumed. Analysis using the finite elementmethod was performed by commercially available software (ANSYS 11.0 fromANSYS, Inc.). The result of analysis shows that the initial displacementamount of the cells on the outermost periphery (endmost cells) is largerthan those of the other cells when a dummy cell is not provided. Theanalysis result also shows that the initial displacement amount issubstantially equal among the cells when dummy cells are provided at theends. Accordingly, it was verified that the variation in initialdisplacement amount of the membrane among the cells is reduced byforming dummy cells around the peripheries of the cells on the outermostperiphery of the element.

FIGS. 10A and 10B show the result of comparison between the initialdisplacement amounts of the center cell and the cell on the outermostperiphery of the element. FIG. 10A shows the result provided when thedepth of the cavity of the dummy cell is 0 (no dummy cell) and thedifference in initial displacement amount between the center cell andthe cell on the outermost periphery of the element is 1. In this case,the width of the cavity of the dummy cell is fixed in all condition.FIG. 10B shows the result provided when the width of the cavity of thedummy cell is zero (no dummy cell) and the difference in initialdisplacement amount between the center cell and the cell on theoutermost periphery of the element is one. In this case, the depth ofthe cavity of the dummy cell is fixed in all conditions. Here, the widthof the cavity of the dummy cell refers to the length of the cavity in adirection parallel to the plane where the cells are arranged, and thedepth of the cavity refers to the length of the cavity in a directionperpendicular to the plane where the cells are arranged.

FIG. 10A shows that the difference in initial displacement amount of themembrane decreases as the depth of the dummy cell increases.Particularly when a dummy cell having a cavity deeper than that of anormal cell is used, there is a possibility that the difference ininitial displacement amount of the membrane between the center cell andthe cell on the outermost periphery of the element can become zero.Further, FIG. 10B shows that the difference in initial displacementamount of the membrane decreases as the width of the dummy cellincreases. In particular, even when a narrow dummy cell having a widththat is 40 percent of the width of the other cells is used, variation ininitial displacement amount of the membrane can be reduced by 90percent, compared with the case in which a dummy cell is not provided.In addition, even when the dummy cell has a width that is 10 percent ofthe width of the other cells, variation in initial displacement amountof the membrane can be reduced by 60 percent, compared with the case inwhich a dummy cell is not provided. Hence, to reduce the variation ininitial displacement amount of the membrane, it is preferable that thewidth of the dummy cell be 10 percent or more of the width of the othercells, and more preferable that the width of the dummy cell be 40percent or more of the width of the other cells.

First Embodiment

A first embodiment of the present invention will be described below. Inthe first embodiment, dummy cells have a width smaller than that ofcells, and a depth equal to that of the cells.

Referring to FIGS. 1A and 1B, an element 101 of the first embodimentincludes a plurality of cells 102 arranged in a plane. Each cell 102includes a membrane 103, a cavity 105 provided in an insulating layer104, an upper electrode 106, and a lower electrode 107. The upperelectrode 106 and the lower electrode 107 are connected electrically. Inthe element 101, all the upper electrodes 105 are electrically connectedby lines 108, and the lower electrodes 107 are electrically isolatedfrom one another. Dummy cells 110 are arranged around cells 109 providedon the outermost periphery of the element 101. Similarly to the othercells, each dummy cell 110 includes a membrane 103, an insulating layer104, a cavity 111, an upper electrode 112, and a lower electrode 113.However, to remove the influence of signals generated in the dummy cells110, at least one of the upper electrode 112 and the lower electrode 113is electrically isolated (not electrically connected) from the upperelectrode 106 and the lower electrode 107 of the other cell. As shown inFIG. 1B, the lower electrode 113 of the dummy cell 110 is isolated fromthe lower electrode 107 of the other cell. That is, since the dummycells 110 are not electrically connected to a circuit board, signalsfrom the dummy cells 110 are not used in subsequent signal processing.

In the first embodiment, since the width of the cavities 111 of thedummy cells 110 is smaller than that of the cavities 105 of the othercells 102, it is possible to suppress the decrease in the effective areaof the element, that is, the ratio of the area of the cells to the areaof the element. Further, since the depth of the cavities 111 of thedummy cells 110 is equal to that of the cavities 105 of the other cells102, the dummy cells 110 can be produced together with the other cells102. Hence, the number of unnecessary processes is not increased.

When the thickness of the upper electrodes 106 can be regarded assufficiently smaller than that of the membranes 103, that is, when therigidity of the upper electrodes 106 is regarded as sufficiently lowerthan that of the membranes 103, the upper electrodes 112 may be omittedfrom the dummy cells 110, as shown in FIGS. 2A and 2B. Similarly, whenthe thickness of the lower electrodes 107 can be regarded assufficiently smaller than the thickness from the bottoms of theinsulating layers 104 to the bottoms of the cavities 105, the lowerelectrodes 113 may be omitted from the dummy cells 110. In second andsubsequent embodiments, upper and lower electrodes are not provided indummy cells so that the arrangement of the dummy cells can be recognizedeasily.

Second Embodiment

A second embodiment of the present invention will be described below. Inthe second embodiment, the depth of cavities of dummy cells is largerthan that of cells.

As shown in FIG. 3, in the second embodiment, the depth of cavities 111of dummy cells 110 is lamer than that of cavities 105 of cells 102.Since this enhances the effect of the dummy cells 110 for reducingvariation in initial displacement amount of the membrane, the decreasein effective area of the element can be further suppressed, comparedwith the above-described first embodiment.

Third Embodiment

A third embodiment of the present invention will be described below. Inthe third embodiment, a plurality of dummy cells are arranged from theinner peripheral side toward the outer peripheral side of the element.

Referring to FIGS. 4A and 4B, two dummy cells 114 are provided outsideeach cell 109 provided on the outermost periphery of an element 101 onthe left side of the figures, and are arranged from the inner peripheralside toward the outer peripheral side of the element 101. In this case,when dummy cells having cavities wider than those of the cells areproduced, the decrease in rigidity of the membrane is suppressed, andthe membrane is prevented from damage.

Fourth Embodiment

A fourth embodiment of the present invention will be described below.

In the fourth embodiment, the present invention is applied to an elementin which cells have a shape different from the square shape (shape inthe top view of the cells) and are arranged in a pattern different froma grid pattern. For example, as shown in FIG. 5, semicircular narrowdummy cells 116 are arranged in an element 115 including circular cells.Alternatively, dummy cells may have the same circular shape as that ofthe cells.

In an element 117 in which cells are arranged in a honeycomb pattern, asshown in FIG. 6, dummy cells 118 are arranged outside cells provided onthe outermost periphery of the element 117 along three axes alpha, beta,and gamma in the figure.

Fifth Embodiment

With reference to FIGS. 7A to 7F, a description will be given of aproduction method for a CMUT including dummy cells having the same depthas that of cells, as in the above-described first embodiment. Thisproduction method is based on the CMUT production method disclosed inU.S. Pat. No. 6,958,255. FIGS. 7A to 7F correspond to the followingsteps (a) to (f), respectively.

(a) Silicon oxide layers 202 and 203 are respectively formed on oppositesurfaces of a SOI (Silicon On insulator) substrate 201.

(b) Through holes 204 are formed in portions of the silicon oxide layer202 where cavities of cells and cavities of dummy cells are to beformed, thereby forming a device substrate 205.

(c) A silicon oxide layer 210 is formed on an upper surface of a throughline substrate 209 including a lower electrode 206, a through line 207,and a pad 208.

(d) The portion of the silicon oxide layer 202 remaining on the devicesubstrate 205 is joined to the silicon oxide layer 210 on the uppersurface of the through line substrate 209.

(e) The layers other than the silicon oxide layer 202 of the devicesubstrate 205 and a device layer 211 of the SOI substrate 201 areremoved to form upper electrodes 212 on an upper surface of the devicelayer 211.

(f) The pad 208 on a lower surface of the through line substrate 209 isjoined to a pad 214 on an upper surface of a circuit board 213.

Since cavities of dummy cells are formed simultaneously with formationof cavities of cells in the above step (b), the CMUT of the presentinvention can be produced through the same number of steps as thatadopted in the production method of the related art. By adding thefollowing steps to the above-described production method, a CMUT inwhich the depth of cavities of dummy cells is larger than that of cells,as in the above-described second embodiment, can he produced. Morespecifically, after the device substrate 205 and the through linesubstrate 209 are formed through the above-described steps (a) to (c),recesses 215 having a depth equal to the difference between the desireddepth of cavities of dummy cells and the depth of cavities of cells areformed in the silicon oxide layer 210 on the upper surface of thethrough line substrate 209, as shown in FIG. 8A. Then, steps similar tothe above-described steps (d) to (f) are performed to produce a CMUTshown in FIG. 8B.

While the present invention has been described with reference toexemplary embodiments, it is to be understood that the invention is notlimited to the disclosed exemplary embodiments. The scope of thefollowing claims is to be accorded the broadest interpretation so as toencompass all such modifications and equivalent structures andfunctions.

This application claims the benefit of Japanese Patent Application No.2008-330366, filed Dec. 25, 2008, which is hereby incorporated byreference herein in its entirety.

1. An electromechanical transducer including an element, wherein theelement comprises: a plurality of cells each including a first electrodeand a second electrode provided with a cavity being disposedtherebetween, the cells being electrically connected in parallel to forma unit; and a plurality of dummy cells provided around an outerperiphery of the unit of the cells, the dummy cell not beingelectrically connected to the cells, wherein a width of a cavity of eachof the dummy cells is smaller than a width of the cavity of each of thecells.
 2. (canceled)
 3. The electromechanical transducer according toclaim 1, wherein a depth of a cavity of each of the dummy cells is equalto a depth of the cavity of each of the cells.
 4. The electromechanicaltransducer according to claim 1, wherein a depth of a cavity of each ofthe dummy cells is larger than a depth of the cavity of each of thecells.
 5. The electromechanical transducer according to claim 1, whereina plurality of the dummy cells are arranged from an inner peripheralside toward an outer peripheral side of the unit of the cells.
 6. Aproduction method for an electromechanical transducer including anelement having a plurality of cells each including a first electrode anda second electrode provided with a cavity being disposed therebetween,the cells being electrically connected in parallel to form a unit,wherein the production method comprises the step of: forming a pluralityof dummy cells around an outer periphery of the unit of the cells, thedummy cell not being electrically connected to the cells, wherein thedummy cells are formed such that a width of a cavity of each of thedummy cells is smaller than a width of the cavity of each of the cells.7. The electromechanical transducer according to claim 1, wherein awidth of each of the dummy cells is 10% or more and 40% or less of awidth of each of the cells.
 8. The production method according to claim6, wherein the dummy cells are formed such that a depth of a cavity ofeach of the dummy cells is equal to a depth of the cavity of each of thecells.
 9. The production method according to claim 6, wherein the dummycells are formed such that a depth of a cavity of each of the dummycells is larger than a depth of the cavity of each of the cells.
 10. Theproduction method according to any one of claims 6, wherein the dummycells are formed such that a plurality of the dummy cells are arrangedfrom an inner peripheral side toward an outer peripheral side of theunit of the cells.
 11. The production method according to any one ofclaims 6, wherein the dummy cells are formed such that a width of eachof the dummy cells is 10% or more and 40% or less of a width of each ofthe cells.